The Milagro GammaRay Observatory

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The Milagro Gamma-Ray Observatory
James Linnemann and Aous Abdo Department of
Physics and Astronomy, Michigan State University
Milagro, the Spanish word for miracle, is a new
type of astronomical telescope. Like conventional
telescopes, Milagro is sensitive to light but the
similarities end there. Whereas "normal"
astronomical telescopes view the Universe in
visible light, Milagro "sees" the Universe at
very high energies. The "light" that Milagro sees
is in the TeV Range.
What is Milagro?
Milagro is a water Cherenkov extensive air shower
(EAS) detector located near Los Alamos, NM at
2630m above sea level, consisting of a 5,000 m2
central (pond) detector surrounded by an array of
175 instrumented water tanks, (outriggers) that
span an area of roughly 40,000 m2. The Milagro
detector has 723 photomultiplier tubes (PMTs)
submerged in a 24 million liter water reservoir.
The PMTs are arranged in two layers, each on a
2.8 x 2.8 m grid. The top layer of 450 PMTs
(under 1.4 m of water) is used primarily to
reconstruct the direction of the air shower. By
measuring the relative arrival time of the air
shower across the array, the direction of the
primary cosmic ray can be reconstructed with an
accuracy of roughly 0.75o.The bottom layer of 273
PMTs(under 6 m of water) is used primarily to
discriminate between gamma-ray Initiated air
shower and hadronic air showers. The sides of the
reservoir are sloped (21) so that the area of
the bottom of the reservoir is smaller than the
top, leading to the smaller number of PMTs in the
bottom layer.
Why Milagro?
When one views the heavens in the TeV range the
picture is quite different from what we see when
we look up at the night sky. The number of
objects we see are much fewer and much more
"extreme".  We see super massive black holes and
neutron stars. Some of these sources are known to
be highly variable, flaring on timescale of
minutes to days. In addition we hope to discover
new sources of TeV photons, possibly observe TeV
emission from Gamma-Ray Bursts, discover
primordial black holes, or discover completely
new phenomena. Until the advent of Milagro there
was no instrument capable of continuously
monitoring the entire overhead sky in the TeV
energy regime. The existing instruments had to be
pointed at small regions of the sky (usually
known sources) and could only look at a source
during the time of year it was overhead at night.
Even then they could only look at the source if
the weather was good and the moon was set.
Milagro is ideally suited to monitor the variable
TeV Universe and discover new sources of TeV
gamma rays.
Arial view of the Milagro detector.
Bottom-layer tubes
Top-layer tubes
The Milagro pond with the cover inflated for
servicing.
Cosmic Rays and Extensive Air Showers
The Earth is immersed in a "sea" of high-energy
nuclei known as cosmic rays. Cosmic rays are
composed of all nuclei, from the simple hydrogen
nucleus (a proton) to the iron nucleus and beyond
(transuranic elements have been observed in
cosmic rays). The energy spectrum of cosmic rays
has been measured up to 109 TeV. When a
high-energy cosmic ray enters the atmosphere it
loses its energy via interactions with the nuclei
that make up the air. At high energies these
interactions create particles. These new
particles go on to create more particles, etc.
This multiplication process is known as a
particle cascade. This process continues until
the average energy per particle drops below about
80 MeV. At this point the interactions lead to
the absorption of particles and the cascade
begins to die. This altitude is known as shower
maximum. The particle cascade looks like a
pancake of relativistic particles traveling
through the atmosphere at the speed
of light. Though the number of particles in the
pancake may be decreasing, the size of the
pancake always grows as the interactions cause
the particles to diffuse away from each other.
When the pancake reaches the ground it is roughly
100 meters across and 1-2 meters thick. If the
primary cosmic ray was a photon the pancake will
contain electrons, positrons, and gamma rays. If
the primary cosmic ray was a nucleus the pancake
will also contain muons, neutrinos, and hadrons
(protons, neutrons, and pions). The number of
particles left in the pancake depends upon the
energy of the primary cosmic ray, the observation
altitude, and fluctuations in the development of
the shower.
AGN
Shadow of the Moon
Active galaxies emit radiation over the entire
electromagnetic spectrum from radio waves to TeV
gamma rays. Thermal emission emanates from the
accretion disk (infrared to X-rays) and the torus
(infrared). Non-thermal emission (radio and
gamma rays) comes from the jets. One of the more
exciting discoveries of the 1990s has been the
observation of TeV emission from several AGNs.
TeV emission has been observed from Mrk 421, Mrk
501, and 1ES2344514, 1ES195965 . Mrk 501 and
1ES2344514 are the first gamma-ray sources
discovered by ground-based instruments. Milagro
data was used to study Mrk 421 while it was
flaring during the period of January to April of
2001 and again in November of 2002. During the
2001 period we observed a 4.7s excess and during
the 2002 flare a 3s excess.
The shadow of the Moon in cosmic rays can be used
to determine the performance characteristics of
Milagro. At TeV energies the Moons shadow is
offset from the actual position of the Moon
because the cosmic rays are bent in the earths
magnetic field. From the position and shape of
the observed shadow one can determine the angular
resolution of the detector and the absolute
energy response of the detector.
The Shadow of the Moon as observed by Milagro.
Mrk 421 during the 2001 flare.
All-Sky Survey
In a manner identical to that used to analyze
data from the region of the Crab nebula, the
entire sky is searched for excesses over the
background cosmic rays using data from Dec. 15,
2000 to Dec. 15, 2001. The sky is binned into
0.1x0.1 degree bins and the expected background
and actual number of events detected in each bin
is determined. These small bins are then summed
into larger bins, commensurate with the angular
resolution of Milagro. The resulting sky map is
shown in the Figure. The circles are drawn around
26 active galaxies identified in Costamante and
Ghisellini 2002 as likely sources of TeV gamma
rays, including the five, which have all been
observed at TeV wavelengths by other
observatories the Crab nebula, Mrk 421, Mrk 501,
1ES1426428, and 1ES2344514 . The brightest
point in the TeV sky over this time period was
Mrk 421. Most of the observed signal in this data
set came from an outburst that began in December
of 2000 and lasted for several months. The next
brightest point in the sky is not associated with
any of the drawn circles and is to the northwest
of the Crab.
Map of the Northern sky in TeV gamma rays. The
scale is the significance of each point in the
sky. The circles mark the locations of AGN and
known TeV sources. Mrk 421 is the brightest
object in the sky over this data set.
The Crab Nebula
The Crab nebula was the first source convincingly
detected in TeV gamma rays . Since the original
detection in 1989 the Crab has become the
standard candle of TeV astronomy. The luminosity
of the Crab is constant (within the accuracy of
the measurements made to date) at 2.68(0.42stat
1.4sys)x10-7 (E/1TeV)-2.59 m-2 s-1 TeV-1. As a
standard candle it is useful for cross
calibrating the sensitivity of different
instruments. From the shadow of the Moon and
Monte Carlo simulation of the detector the
angular resolution of Milagro is 0.8 degrees. The
square angular bin that maximizes the
significance of a signal has a width 2.8 times
the angular resolution of the detector .
Therefore an angular bin of width 2.1 degrees is
used in this analysis.
Data taken in the Crab Nebula region with 6 s in
the position of the Crab.
The Galactic Plane
Diffuse emission from the galactic plane is the
dominant source in the MeV gamma ray sky. Milagro
detected, for the first time, the galactic plane
in the TeV range. The emission seems to be
concentrated in the Cygnus region
Other topics that we are currently studying
include the study of Gamma Ray Bursts Solar
physics and Dark Matter. The Milagro
collaboration consists of more than ten
institutions. To date more than ten Ph.D. theses
have been completed.
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